In order to provide for the direct injection of fuel into an engine combustion chamber, high fuel pressures are required to overcome compression pressures in the chamber and to generate very fine fuel atomization. The injector must solely prepare the fuel for combustion since the mixing of air and fuel must take place in the combustion chamber during the compression stroke. The time for injection of fuel is limited to the period after the intake valve is closed up to just before the point of ignition. These requirements are considerably more demanding than those of current common systems using port fuel injection. Required fuel pressures for direct injection are on the order of 1500 PSI and fuel particles prior to combustion should be in the range of 15 micrometers or less. The window or time for injection is about 1/4 of that for port fuel injection and thus requires a dynamic range (and static flow rate) which is about four times that of a typical port fuel injector.
Direct injection (DI) injectors must be located in the cylinder head. Prior embodiments of DI injectors have generally been larger than current port fuel injectors making it extremely difficult to mount them without compromising the engine cylinder head.
To achieve fast operation at high fuel pressures, DI injectors have sometimes used high voltage and current for actuating their solenoid driven valves. Such voltages and power levels are difficult and expensive to achieve with vehicle systems based on 12 volt DC electrical systems. Further, DI injectors have often been limited as to dynamic range. Consequently, engines provided with such injectors have not been able to run at both low and high load levels.
Typically, DI injectors have used inwardly opening pintle valves in combination with a fuel swirler. The fuel travels through the swirler and then through a single orifice before creating a spray. The fuel recombines in this orifice before the spray is created, making it difficult to achieve small particles as desired. Other DI systems have used outwardly opening pintle nozzles, relying on a pressurized air source to break up the fuel into small droplets. Such systems require an air pump and an additional actuator.
Inwardly opening pintle-type injectors may be affected by combustion chamber deposits which form in the exit orifice, disturbing the fuel spray and decreasing the flow rate. Further, combustion pressures can force a fuel valve to open if the fuel pressure is low and the pintle spring rate is low. Back flow from the combustion chamber can force particles into the injector, upsetting the spray formation and possibly sticking the injector open. Increasing spring load to insure that the injector won't allow back flow, adversely affects opening time as the actuator must overcome this load to open the injection valve.